COACTIVATION OF STERNOCLEIDOMASTOID MUSCLES DURING MAXIMUM CLENCHING

In an attempt to determine the degree of co-activation present in sele cted cervical muscles during clenching, we instructed 12 male subjects to produce four brief maximum voluntary contraction (MVC) efforts (cl enching) in a position of maximum intercuspation. Surface EMG activity was recorded bilaterally from the masseter and sternocleidomastoid (S CM) muscles. The contraction level for the SCM during clenching was re ported as a percentage of the SCM's maximum activity achieved during m aximum neck flexion against resistance. All EMG signals for the masset er and SCM were converted to a true RMS voltage signal and digitized a t a 100-Hz sampling rate. Mean peak EMG voltage levels were determined for the activity recorded during each brief MVC task. All subjects de monstrated co-activation of the SCM during strong abrupt clenching eff orts. The mean levels (+/- S.D.) of SCM activity were 11.8 +/- 9.6% (r ight) and 14.2 +/- 9.4% (left) of the MVC capacity. Fifty percent of m asseter activity was required to achieve 5% activity of the SCM bilate rally, and there was a progressive development of the SCM co-activatio n which paralleled the masseter activation.